I searched, I promise - 901 tranny weaknesses, can they be cured?

[kenikh]

We all know 1st gear is weak on the 901, 2nd is only a little better and its shifting manners in these lower gears can be a little balky. Yet that shift pattern and other charms make me swear I will never go to a 915, even if it has all the Wevo doo-dads.

How can one build a 901 that can be abused (relatively). We all know you can buy a shifter kit and precision shift joint to sort any shifting issues. I have also read that a billet aluminum intermediate plate and 904 mainshaft upgrades can help regarding internal weaknesses, but all of that info is completely anecdotal. Has anyone out there built a 901 that can handle a 2.8 RSR by laughing in its face?

If you have, tell us your secrets.

[SandyI]

Long Live the 901!

Personally, I'd call Otto @ Otto's Venice to have a professionally built 901 to handle all that HP. I think his personal tube frame racer runs 350 hp through a 901. Your 901 won't be cheap but it will kick some serious ass.

[kenikh]

I'd be curious to call him and see what he knows. That sounds like one heck of a 901.

BTW, I love Topanga Canyon; my bro-in-law lives in one of the canyons up around Chatsworth and I try to abuse your roads as much as possible when down there.

[randywebb]

the 1st gear design is inherently weak and the trans was just not designed for a whole lotta hp, but as you noted those items will help -- "cure" is a loaded word...

search for some of Steve Wiener's posts on this and get his opinion too

avoid drag racing Mopar hemis...

[Grady Clay]

First, the 901 is a remarkably good transmission. It is a very worthy successor to the 741 4-speed and 718 5-speed. There are limits. The transmission was sized for engines in the 2.0 to 2.2 range with appropriate safety margins. The issue of sizing includes gear width, center-to-center distance between the input shaft and the pinion shaft, the bearing design & location, the ring & pinion size and tooth design, and the strength of the castings.

Porsche chose to locate the 1st gear as a cantilevered “starting gear” just like the 718. The flaw in this approach is when someone does “jack rabbit” starts in first gear with current sticky tires. Remember, the design was cast in stone (well, aluminum) in about 1961 when the best race tires wouldn’t nearly approach the grip of current street tires.

Porsche recognized the problem and incorporated the solution in the design of the 915. The cantilevered 5th gear can only see engine torque and very little inertial load when shifting. The 915 5th gear only sees up-shift loading. A 901 first can see loads both directions and huge loads when the revs aren’t matched. In about ’67 Porsche “reinforced” the input shaft by making the threads larger (M12 to M14) on the 1-R extension. The replacement part 904 input shaft got the M14 threads in the late ‘60s.

The other design decision that was less than perfect was to locate the 4-point ball bearing in the intermediate plate and not closer to the pinion gear. Using only four bearings (two on the input shaft and two on the pinion shaft) they used the stronger roller bearing to support the huge lateral loads from the pinion gear and the 4-point bearings to support the axial loads and the smaller lateral loads. The 915 addressed this by moving the 4-point bearings toward the pinion gear and adding a pair of roller bearings (now total of six) adjacent to the 4-point bearings. Even these were enlarged in the latest 915s.

The only solution to satisfactory longer life 4-point bearings is to replace them with new at each opportunity. The failure mode is bernuling (sp?) of the inner race where the harder metal breaks off in small pieces. This is greatly exacerbated if someone uses 4-5 gears flipped for the 3rd gear position or vise versa.

The other two big changes in the 915 design was to increase the shaft separation and make the gears wider. All this gave the 915 a significant increase in capacity for 2.4, 2.7, 3.0 and 3.2 engines but the price was weight.

While I am a fan of the light weight magnesium (Electron) castings used in late ’69 901, ’70-’71 911, 914, 914-6, and early 915s, the material strength isn’t as great as aluminum. This becomes an issue with higher torque and simple age. There is an aluminum aftermarket replacement intermediate plate available for the magnesium 901s. I don’t think there ever was a strength-of-materials need for an aftermarket intermediate plate for the aluminum 901.

I would recommend the F (18:34) or GA (18:32) second gear cut on the input shaft over the very expensive 904 input shaft and 2nd gear only splined gear sets. These are stronger gears/shafts, just not as easy to change ratios between track sessions. There was also a subtle change in ’70 with slightly (0.8 mm) wider 2nd gear.

I have never felt that the 901 and ’72-> 914 had any issue with shifting. With everything working and adjusted properly, they are great.

I use a ‘71 Type 911 transmission for my 2.8S engines. I try and be not the slightest bit abusive.

For an autoX transmission, I would prefer an aluminum 915 4-speed or 930 4-speed with appropriate ratios.

Best,
Grady

[donstevens]

Great response Grady.

kenikh, per Grady's advice, "I use a ‘71 Type 911 transmission for my 2.8S engines. I try and be not the slightest bit abusive", you can probably use a fresh 901 and be OK. Otherwise, upgrade to a 915.

I have a good firend who had a 67S vintage race car with a 2.6 and a 901. We drove it real hard, actually won an HSR 3 hr enduro, and it lasted about a full season before the tranny gave up. That being the measure, you'll probably be OK in a street car.

Don